The present invention relates to a counting device which counts the number of signals and an interferometric distance meter which obtains the distance to a measurement target from which the number of interference waveforms has been measured by using the counting device.
Distance measurement by a laser using optical interference does not disturb a measurement target because of noncontact measurement, and has been used for a long time as a high-accuracy measurement method. Recently, attempts have been made to use a semiconductor laser as a light measurement light source to achieve a reduction in apparatus size. A typical example of such an apparatus is an apparatus using an FM heterodyne interferometer. This apparatus can measure a relatively long distance with high accuracy, but has a drawback of a complicated optical system because of the use of an interferometer outside a semiconductor laser.
In contrast to this, a measuring instrument has been proposed, which uses the interference between output light from a semiconductor laser and optical feedback from a measurement target inside the laser (self-mixing effect, self-coupling effect). The laser measuring instruments using self-mixing effect are disclosed in, for example, reference 1 (Tadashi Ueda, Jun Yamada, and Susumu Shitoh, “Distance Meter Using Self-Mixing Effect of Semiconductor Laser”, Papers for 1994 Tokai-Section Joint Conference of the 8 Institutes of Electrical and Related Engineers), reference 2 (Jun Yamada, Susumu Shitoh, Norio Tuda, and Tadashi Ueda, “Study of Compact Distance Meter by Self-Coupling Effect of Laser Diode”, Bulletin of Aichi Institute of Technology, Vol. 31B, pp. 35-42, 1996), and reference 3 (Guido Giuliani, Michele Norgia, Silvano Donati and Thierry Bosch, “Laser diode self-mixing technique for sensing applications”, JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS, pp. 283-294, 2002).
In such a laser measuring instruments using self-mixing effect, a photodiode built-in semiconductor laser has light-emitting, interference, and light-receiving functions at the same time, and hence allows great simplification of an external interference optical system. A sensor unit therefore comprises only a semiconductor laser and a lens, and becomes smaller than conventional sensor units. This instrument also has a characteristic feature that its distance measurement range is wider that of triangulation.
FIG. 23 shows a complex resonance model of an FP type (Fabry-Perot type) semiconductor laser. Referring to FIG. 23, reference numeral 101 denotes a semiconductor laser; 102, a cleavage plane of a semiconductor crystal; 103, a photodiode; and 104, a measurement target. Part of reflected light from the measurement target 104 tends to return into the oscillation area. Slight optical feedback mixes with laser beam inside the resonance 101, resulting in unstable operation and noise (complex resonance noise or optical feedback noise). Even a very small amount of optical feedback relative to output light causes a noticeable change in the characteristics of the semiconductor laser. Such a phenomenon is not limited to a Fabry-Perot type (to be referred to as an FP type) semiconductor laser, and also occurs in other types of semiconductor lasers such as a vertical cavity surface emitting laser type (to be referred to as a VCSEL type hereinafter) and a distributed feedback laser type (to be referred to as a DFB laser type).
Let λ be the oscillation wavelength of the laser and L be the distance from the cleavage plane 102 near the measurement target 104 to the measurement target 104. In this case, when the following resonance condition is satisfied, optical feedback and laser beam in the resonance 101 strengthen each other. Consequently, the laser power slightly increases.L=nλ/2  (1)where n is an integer. This phenomenon can be sufficiently observed even with very weak scattered light from the measurement target 104 when an amplifying action occurs as the apparent reflectance inside the resonance 101 of the semiconductor laser increases.
A semiconductor laser emits laser beam having different frequencies in accordance with the magnitude of injection current. This laser therefore allows direct modulation of the oscillation frequency by using an injection current without requiring any external modulator. FIG. 24 shows the relationship between the oscillation wavelength and the output waveform of the photodiode 103 when the oscillation wavelength of the semiconductor laser is changed at a predetermined rate. When L=nλ/2 indicated in equation (1) is satisfied, the phase difference between optical feedback and laser beam inside the resonance 101 becomes 0° (in phase), and the optical feedback and the laser beam inside the resonance 101 strengthen each other most. When L=nλ/2+λ/4, the phase difference becomes 180° (in opposite phase), and the optical feedback and the laser beam inside the resonance 101 weaken each other most. As the oscillation wavelength of the semiconductor laser is changed, therefore, the laser power increases and decreases alternately and repeatedly. When the laser power is detected at this time by the photodiode 103 provided in the resonance 101, a staircase waveform having a constant period like that shown in FIG. 24 is obtained. Such a waveform is generally called an interference pattern.
Each of the elements of this staircase waveform, i.e., the interference pattern, is called a mode hop pulse (to be referred to as an MHP hereinafter). MHP is a phenomenon different from a mode hopping phenomenon. Assume that the distance to the measurement target 104 is represented by L1, and the number of MHPs is 10. In this case, as the distance decreases to L2 which is ½ of L1, the number of MHPs becomes five. That is, as the oscillation wavelength of the semiconductor laser changes in a predetermined time, the number of MHPs changes in proportion to the measurement distance. Therefore, detecting MHPs by the photodiode 103 and measuring the frequency of MHPs can easily measure the distance.
Conventional interference type distance meters including self-mixing type distance meters obtain the distance to a measurement target by measuring the number of MHPs using a counting device or by measuring the frequency of MHPs using FFT (Fast Fourier Transform).
In a distance meter using FFT, however, if a change in the oscillation wavelength of the laser is not linear with respect to time, the peak frequency calculated by FFT differs from the average frequency of MHPs which should be obtained, resulting in an error in the measured distance.
In a distance meter using a counting device, if noise such as disturbance light is counted as an MHP or there is an MHP which is not counted due to the pulse omissions of a signal, an error occurs in the number of MHPs counted by the counting device. As a result, an error occurs in the measured distance.
Note that such a count error is not limited to distance meters and may occur in other counting devices in the same manner.